Combustible (flammable) gas sensors have been in use for many years for the prevention of accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases. Conventional gas sensors typically comprise a platinum wire assembly encased in a refractory (for example, alumina) bead, which is impregnated with a catalyst. This encased assembly is commonly referred to as a pelement or a pellistor. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987).
In general, the pelement operates as a small calorimeter which measures the energy liberated upon oxidation of a combustible gas. The platinum element serves two purposes within the pelement: (1) heating the bead electrically to its operating temperature (typically approximately 500.degree. C.) and (2) detecting the rate of oxidation of the combustible gas.
The rate of oxidation of the combustible gas may be measured in terms of the variation in resistance of the platinum element relative to a reference resistance. The two resistances are generally part of measurement circuit such as a Wheatstone bridge circuit. The output or the voltage developed across the circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The reference resister generally comprises a compensating, nonactive pelement having chemical and physical characteristics matched as closely as possible with the pelement carrying the catalyst.
Typically, the active pelement and the compensating pelement are deployed within an explosion-proof housing and are separated from the surrounding environment by a porous metal frit. The porous metal frit allows ambient gases to pass into the housing but prevents the "flashback" of flames into the surrounding environment. Such catalytic gas sensors are usually mounted in instruments which, in some cases, must be portable and, therefore, carry their own power supply. It is, therefore, desirable to minimize the power consumption of a catalytic gas sensor.
In recent years, substantial research effort has been devoted to the development of combustible gas detectors using semiconductor technology and silicon micromachining. Although the typical electrical power dissipation of conventional catalytic gas sensors is on the order of 250 to 700 mW, miniature, integrated catalytic gas sensors having electrical power consumption on the order of 100 mW and less are under development. See Krebs, P. and Grisel, A., "A Low Power Integrated Catalytic Gas Sensor," Sensors and Actuators B, 13-14, 155-158 (1993).
In general, the overall electronic control circuit design of these microsensors is very similar to that of conventional combustible gas sensors. Referring to FIG. 1, microsensor 10 is provided with both a catalytically active element 20 or detector and a catalytically inactive compensating element 30 or compensator, each of which is used in a measurement circuit such as a Wheatstone bridge circuit. Detector 20 and compensator 30 are disposed upon a microheater chip 40, which is disposed upon a substrate 50.
While the feasibility and advantages of microfabricated sensors have been demonstrated, microfabrication of practical combustible gas microsensors has yet to be achieved. See Wu, Q. et al., "Development of Chemical Sensors Using Microfabrication and Micromachining Techniques," Sensors and Actuators B, 13-14, 1-6 (1993). For example, a significant problem to be solved in the commercialization of microminiature combustible gas sensors is the deposition or application of a suitable catalyst upon the microheater chip to form the detector.
Material application techniques in the semiconductor arts are typically categorized as "thick film" techniques or "thin film" techniques. Thick film techniques generally involve material application via a stencil or silk screen technique. The desired material, in a carrier agent or agents, is printed on the target as determined by a pattern of openings cut into the stencil. Carrier agents are removed by drying and the material is occasionally fused on the surface by a baking process. The thick film paste is blended to allow relatively easy processing. The resulting thickness of the applied material depends upon the thickness of the screen or stencil and the percent of solids in the paste. As a result, the thickness of the applied material can range from a thousandth of an inch or less up to 0.125 in. and even greater.
Existing techniques for application of thick films cannot be used to deposit a catalyst film having appropriate dimensions for optimal performance of combustible gas microsensors. The dimensions of the target are too small to accurately align a stencil or silkscreen. Moreover, the pressure required to squeeze the paste (material and carrier agent) through a stencil can break the delicate membranes upon which microsensors are positioned. Finally, the small size of the target footprint will not allow the paste to adhere easily to the surface when the stencil is lifted.
Thin film techniques generally involve material application via vapor deposition techniques, laser evaporation techniques or photolithographic techniques. No carrier agent, silk screen or stencil are required in such techniques. High purity, dense packing and consistent thickness of a few molecules and greater are achievable. In general, the upper limit of the thickness of the applied material obtainable via thin film techniques is approximately 0.001 in. Patterns are created by a photolithographic or similar sacrificial coating technique. Much greater accuracy and resolution are obtainable with thin film techniques than with thick film techniques.
Although catalyst films are presently applied to combustible gas microsensors via thin film techniques as describe above, it has become apparent that thin film techniques cannot achieve a catalyst film of sufficient thickness for acceptable performance of combustible gas microsensors. There are at least three significant problems with the deposition of the catalyst as a thin film. First, the resultant dense packing and consistent film thickness provide a very limited number of catalyst sites and, therefore, a very low signal from the detector. Second, the limited number of catalyst sites causes the detector to have a short life in the presence of even very small amounts of a catalyst contaminant or poison. See e.g., Krebs, P., supra. Third, the dense packing and adhesion to the thin substrate layer characteristic of thin films introduce very high interfacial stresses as the sensor is heated because of differences in thermal coefficients of expansion between the two materials.
It is, therefore, desirable to develop a method of depositing a catalyst system upon a combustible gas microsensor that does no suffer from the above-discussed drawbacks.